1. Field of the Invention
The invention relates to an echo canceller which cancels echo generated at 2-wire/4-wire conversion portion due to the impedance mismatching and a method for controlling the echo cancellation of the echo canceller.
2. Description of the Prior Art
Along with the increasing of communication demand in recent years, communications networks, which transmit integrated signals such as voice, images and data signals using digital multiplexers, are constructed everywhere in the world. For the high efficiency transmission of voice signals, it is usual to equip low-bit-rate CODECs with the digital multiplexers.
Furthermore, in case that the transmission delay becomes significant, because of the long transmission path such as international links or because of the processing delay introduced by the voice CODECs, echo cancellers shall also be equipped with the digital multiplexers.
FIG. 13 is an example of a telephone communication network which connects three points using digital multiplexers. In FIG. 10, elements 1, 2 and 3 are telephone terminals, elements 4, 5 and 6 are exchanges, elements 7, 8 and 9 are digital multiplexers, elements 10 and 11 are full duplex transmission links. When at telephone call from telephone terminal 1 to terminal 2, for example, is originated, only one transmission link and a pair of echo canceller, consequently, are involved within the end-to-end path. But when a telephone call from telephone terminal 1 to terminal 3, for example, is originated, two transmission links, 10 and 11, are connected in tandem by the exchange 5 to constitute an end-to-end transmission path, and two pairs of echo cancellers, consequently, are involved within the path as indicated in FIG. 14.
In such circuit configuration, it is desirable to disable the echo cancellers 13 and 14 which are put in the midst of the path. One reason is that the echo canceller which is put in the midst of the transmission path, echo canceller 13 for example, is not always capable to accommodate itself to the unknown transmission delay which exist across the exchange, where the total echo delay consists of the delays introduced by transmission links, link 11 for example, and the processing delays introduced by the CODECs, those implemented within multiplexers 8 and 9 for example. If the signal path across the exchange is composed of plural links, there is the possibility that the sum of the delays exceeds the adaptability of the echo canceller. Another reason is that the plural echo cancellers in the same path which serve for the same telephone terminal, echo canceller 13 and 15 for telephone terminal 1 for example, may disturb their echo cancelling operations to each other because each operation, which results in the modification of the transmission characteristics of the echo path, is performed independently. For those reasons, the echo path estimation of each echo canceller connected in tandem may be interfered and the overall transmission quality for the voice channel may be deteriorated.
In the past, for example, the laid-open Japanese patent publication No. 4-165825/92 discloses an echo canceller which disables echo cancelling function automatically when it is necessary. FIG. 15 shows a block diagram of the echo canceller in the above cited reference.
In FIG. 15, element 17 is an oscillator, element 22 is an echo canceller, elements 19 and 20 are channel associated signalling detectors, element 21 is a switching element which selects the output signal of echo canceller or output signal of oscillator for its output, element 18 is a micro processor which controls the operation of switching element 21 based on the information derived from channel associated signalling detectors 19 and 20, element 23 is a transmitting side input port, element 24 is a receiving side output port, element 25 is a transmission side output port and element 26 is a receiving side input port. The external circuit from element 24 to element 23 is usually called the tail circuit and another external circuit from element 25 to element 26 is usually called the long haul circuit. An echo canceller is designed to cancel the echo signal originated in the tail circuit.
The operation of echo canceller shown in FIG. 15 is as follows. The oscillator 17 generates the 2100 Hz tone signal which corresponds to the echo canceller disabling tone signal as specified in the ITU-T Recommendation G.165. The micro processor 18 senses the status change of channel associated signalling information using the channel associated signalling detectors 19 and 20, and recognizes whether the voice channel is in a busy state or in the idle state. The switch 21 operates under the control of the micro processor 18, normally selecting the input signal from the echo canceller 22 for its output. But for a pre-determined short period after the status change of signalling information from the idle state to the busy state, the switch 21 selects the 2100 Hz tone signal generated by the oscillator 17 for its output. On detecting the 2100 Hz disabling tone, the echo cancellers in the network with automatic disabling capability can disable the echo cancelling function. This invention was made to disable the echo cancelling function within the digital network when the terminal equipment with echo cancelling capability, such as speaker phone terminals or tele-conference terminals are connected to the digital network with echo cancelling capabilities within the network to eliminate the cross interference between the network echo cancelling functions and the echo cancelling function of external terminal equipment.
This invention is valid only when the disabling tone generation capability is implemented with the echo canceller of terminal equipment. If the echo cancellers of above stated invention are implemented within the transmission network, in digital multiplexers for example, there is no way for each echo canceller to recognize whether it is put at the either end of the transmission path, as echo cancellers 12 or 15 in FIG. 13, or put at the midst of the transmission path, as echo cancellers 13 and 14 in FIG. 13 for example. In such case as indicated in FIG. 13 for example, the echo cancelling function of echo cancellers 12 and 15 which have to cancel the echo signals, generated at 4-wire/2-wire conversion elements within the exchanges 4, 6 and also in telephone terminal 1 and 3, are also disabled at the same time and the network performance is deteriorated.